There is substantial evidence that the vitamin K-epoxide cycle is involved in the synthesis of prothrombin. Agents which block either the regeneration of vitamin K from vitamin K epoxide or the epoxidation of vitamin K are anticoagulants. How the cycle is involved in the carboxylation of glutamic acid residues to gamma- carboxyglutamic acid (Gla), a vitamin K dependent step in the synthesis of prothrombin, is not kn wn. The development of in vitro systems which carry out prothrombin synthesis, carboxylation and the vitamin K-epoxide cycle will allow us to determine if these processes are correlated under a variety of conditions. The hypothesis that the site of action of anticoagulants is the vitamin K-epoxide cycle will be further examined. The discovery of Gla in bone and kidney suggests that vitamin K may have a role beyond that in clotting protein synthesis. We will examine bone, kidney and other extra hepatic tissues for the vitamin K-epoxide cycle and determine its relation to protein carboxylation. The inhibition of the cycle and protein carboxylation by anticoagulants will be tested.